Rpdm chapter10

Road Planning and Design Manual Chapter 10: Alignment Design

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Chapter 10

Alignment
Design

August 2001

Chapter 10: Alignment Design Road Planning and Design Manual

Table of Contents

10.1 Introduction 10-1

10.2 Design Principles 10-1

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10.2.1 Introduction 10-1
10.2.2 Driver Perception of Roadway 10-2
10.2.3 Holistic Design Approach 10-3
10.2.4 Aesthetics of Road Design 10-3
10.2.5 Designing for Sun Position 10-5

10.3 Alignment Coordination 10-5
10.3.1 General 10-5
10.3.2 Principles of Co-ordination 10-8
10.3.3 Specific Co-ordination Issues 10-8

10.4 Fitting the Road to the Terrain 10-9
10.4.1 General 10-9
10.4.2 Approaches to Design 10-15

10.5 Curvilinear Design in Flat Terrain 10-19

References 10-22

Relationship to Other Chapters 10-22

August 2001


Chapter 10 Amendments - August 2001

Revision Register

Issue/ Reference Description of Revision Authorised Date
Rev No. Section by
1 First Issue Steering May

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Committee 2000
2 10.2.4 Paragraph added at the end of section
10.3.3 Cross reference to Figure 10.7 added Steering Aug
Figure 10.8 - title added Committee 2001
10.4.1 Corrections to references
New Relationship to Other Chapters

August 2001 iii


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iv August 2001


Chapter 10
Alignment Design
10.1 Introduction • Historic features (including both indigenous
and introduced cultural heritage)
The overall quality and appearance of a road will
• Features to preserve (including natural

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be determined by the quality of the alignment
design (horizontal and vertical) and its environment)
relationship to the surrounding environment. Designers need to employ imagination, ingenuity
“An important concept in highway design is that and flexibility to get the best result.
every project is unique. The setting and character It is important that all of the necessary
of the area, the values of the community, the needs information on both physical constraints and
of the highway users, and the challenges and community expectations is obtained early in the
opportunities are unique factors that designers process. It is also important that the special
must consider with each highway project. expertise of landscape architects, environmental
Whether the design to be developed is for a specialists and others is employed at the
modest safety improvement or 10 miles of new- beginning before controlling decisions are made
location rural freeway, there are no patented and changes can still be adopted.
solutions. For each potential project, designers are
faced with the task of balancing the need of the This chapter is devoted to the design of the
highway improvement with the need to safely horizontal and vertical alignments as a
integrate the design into the surrounding natural coordinated whole in harmony with the landscape,
and human environments.” (Flexibility in the environment and the community in general.
Highway Design (US DOT, 1997).

A holistic approach to the design is required
where the various elements of the road design are 10.2 Design Principles
considered as a whole in the context of the
environment through which the road passes and 10.2.1 Introduction
the expectations of the community affected by the
road. To achieve the best result, designers must Design principles have to consider the entire
consider the engineering requirements of the road range of factors impinging on the design -
in question together with the characteristics of the engineering, landscape, environmental,
area in which the road is located. The designer community expectations. This manual deals in
must consider the character of the area in the detail with the engineering requirements of road
context of its physical location and give proper design, taking into account the other factors to be
weight to: addressed. For a complete treatment of the other
issues, designers should refer to the respective
• Physical characteristics of the area manuals developed for the disciplines involved:

• Use of the corridor (destination spots, • Road Landscape Manual
pedestrians, cyclists, rural industry, urban
residential etc) • Erosion and Sediment Control Manual

• The type and scale of the road to be designed • Road Project Environmental Management
Processes Manual
• Vegetation in the corridor (density, type)
• Road Traffic Noise Management Code of
• Views available, scenic values Practice

August 2001 10-1


In engineering terms, road alignments have to become more alert at a particular point in time
service traffic in terms of providing a route that and not be able to recall the last few minutes of
meets constraints imposed by vehicle dynamics, the trip.
occupant comfort, and topography. This means
that most road alignments are in fact complex 3. Driver fatigue.
three-dimensional splines that do not have a
4. Driver use of drugs and alcohol.
simple mathematical definition. This problem has
historically been addressed by reducing the three- 5. Driver distraction.
dimensional alignment to two two-dimensional

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alignments. In each case, the alignments are made 6. Driving at night/dawn/dusk.
up of geometric elements that are convenient to
7. Driving in rain.
calculate and construct yet still ensure that vehicle
dynamics constraints are met. Even with the 8. Driving in fog.
advent of computers, this approach has still
proven to be the most convenient for both design All of the above factors (except dot point 4) can
and construction. To use complex three- affect most drivers at particular times. This results
dimensional models would introduce unnecessary in instances where drivers have reduced
complexity into the process but it is necessary to perception of the roadway. For this reason, it is
ensure that the two alignments are properly very important to provide drivers with as many
coordinated and complement each other. clues as possible as to what lies ahead. This will
minimise any safety problems especially during
Details of the specific engineering requirements times of reduced driver perception. Roadside
for horizontal and vertical alignment are provided conditions must not be ambiguous or misleading
in Chapters 11 and 12 of this manual. The intent as this provides an inadequate level of driver
of this Chapter is to put those requirements in perception.
context and emphasise the importance of the
complementarity of the design of those elements. The provision of as many clues as possible as to
what lies ahead is especially critical for geometric
elements comprising a large decrease in speed i.e.
10.2.2 Driver Perception of high speed minor road ending at a rural
Roadway intersection. Methods to achieve this are
discussed in Section 8.6.
Drivers use the visual information of the roadway
geometry to develop their perception of the road Particular aspects of geometric design which
conditions and thereby control the speed and provide good/poor driver perception are discussed
position of their vehicles on the roadway. They below.
need sufficient information to allow them to take
the necessary decisions to safely negotiate each • Co-ordinating horizontal and vertical
geometric feature. alignments as discussed in Section 10.3
improves driver perception of the roadway.
The amount of visual information perceived by
drivers is limited, however, by the following: • The smooth curving face of a cutting can
provide good driver perception of the
1. Drivers travelling for relatively long periods of corresponding curvature in a road.
time at high speed become ‘velocitised’ (losing
perspective of speed of travel and physical Conversely, a fill cannot provide good driver
surroundings). perception if the view ahead is open to the sky
where there may be no visual background of
2. Inadequate attention because of driver ‘day fixed objects.
dreaming’ rather than concentrating entirely on
the driving task. In this state, a driver can often Even on flat terrain, a horizontal curve can
provide poor driver perception if there is no

10-2 August 2001


visual background of fixed objects. Using a holistic approach will mean that designers
will critically examine the application of minimum
• Vegetation or other obstructions on the inside requirements in the design. Combinations of the
of sharper curves can provide inadequate driver minimum requirements of complementary
perception of the tightness of the curve. elements will rarely produce a satisfactory result
and in many cases will produce an unsafe result. A
• A driver will not expect a curve if this sole cue
better result will usually be achieved by adopting
comes from a line of service poles or trees
better than the minimum and this can often be
which continue straight on, or from a deceptive
done at the same or lower cost.
gap in a line of trees on the outside of the curve.

• A crest should not obscure a potential hazard
such as a narrow bridge, a railway at-grade 10.2.4 Aesthetics of Road
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crossing, an at-grade intersection or a Design
horizontal curve which requires a significant
reduction of speed. Even small crests can It is important to realise that no road can be
obscure important features. The visual clues regarded as aesthetically successful if it is not
presented to the driver by the view of the road integrated with the natural and man made features
surface are especially important on sections of that surround it. Cosmetic treatments in the form
sharp curvature. of shrubs and trees cannot overcome the aesthetic
deficiencies of poor location and design.

In addition to providing a pleasant visual
10.2.3 Holistic Design Approach
experience to drivers (and possibly because of it),
Designers must take a holistic approach to the accident rates are lower on roads with good
total road design and consider the effects of each aesthetic characteristics. Roads designed to fit the
element on the other elements of the design. The terrain and take advantage of adjacent natural
horizontal and vertical alignments must be features create less driving tension and weariness
considered together and the effect of the cross for the driver.
section must also be considered at this time. The Two perspectives have to be considered in the
approach to alignment coordination is discussed aesthetics of a road:
in 10.3.
• the view of the road; and
An important issue is the size of the road
compared to its surroundings (its scale). The cross • the view from the road.
section has the greatest effect on this - the wider
the road, the greater the scale. In some From the community perspective, the impact of
environments, a large scale may fit well with the the road on their outlook, or the appearance of the
surrounds and be compatible with them. However, road from their residences, is the major factor.
it is often the case that roads will have a scale too This perspective is helped by application of good
large for the environment and the design should design principles as discussed elsewhere in this
attempt to mitigate this effect. Chapter but it may be necessary to provide other
features such as shielding with vegetation and
Some design techniques that will tend to reduce earth mounds to overcome the intrusion of the
the perceived width and therefore the perceived road into the community’s landscape. This is
scale include: beyond the scope of this manual.
• Median planting
The view from the road is within the control of the
• Reducing the extent of shoulder seal designer and application of the design principles
• Independent grading enunciated in this manual should achieve a
pleasing result.
• Landscaping

August 2001 10-3


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Figure 10.1 Well designed roads in harmony with their surroundings produce a pleasing appearance

10-4 August 2001


It is desirable that any point of interest in the 10.2.5 Designing for Sun
vicinity of the road (e.g. areas of high scenic Position
value) be able to be viewed by users of the road.
The sub-section titled “Development and The impact of the sun shining directly into a
Maintenance of Views”, Section 3.4.2 of this driver's eyes should be considered in the location
Manual, discusses this issue. and design of a road. It is not always possible to
design a road to avoid this problem in all
circumstances but careful consideration should be
given to it particularly when conflict situations

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occur. An intersection on the crest of a hill is one
such case.

Judicious application of curvature, subtle changes
Undulating mound in direction and provision of appropriate
landscaping can help to alleviate some of the
situations where sun glare is a problem. The
different sun positions throughout the year make
the analysis more complex but they have to be
considered. Figures 10.3 and 10.4 provide solar
charts for Queensland and provide a guide for
designers in assessing the impact of sun position
on a road design.

10.3 Alignment Coordination

Narrow mounds 10.3.1 General

The importance of alignment coordination cannot
be over emphasised. The appearance, economy
and safety of the road design depend on the
successful achievement of proper coordination.
Details of the individual elements of horizontal
and vertical alignment design are provided in
Terraced mound Chapters 11 and 12 - this Chapter will address
their combination.

Coordination of alignment and profile should not
be left to chance but should begin at the concept
stage, during which adjustments can readily be
made. The designer should study long, continuous
stretches of highway in both plan and profile and
visualize the whole in three dimensions. (See
AASHTO, 1994).

If long lengths are not considered, it is possible to
produce discontinuities in appearance even
Retained mounds though the shorter lengths, taken in isolation, are
satisfactory. (See upper photograph in Figure
10.11).
Figure 10.2 Landscape Mounding Alternatives

August 2001 10-5


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The centre of the diagram represents the observer’s position. The heavy curved lines represent the sun’s
path for selected dates and latitude 27.5° South and are crossed by lines indicating hours. To find the
sun’s position for the required conditions, select the point where the appropriate lines intersect. The sun’s
altitude (in degrees above the horizontal plane) is shown by the relation of this point to the concentric
circular lines within the diagram. The direction of the sun’s rays is shown by a line drawn through this point
from the outer graduated circle towards the centre.

Figure 10.3 Solar Chart for South Queensland

10-6 August 2001


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The centre of the diagram represents the observer’s position. The heavy curved lines represent the sun’s
path for selected dates and latitude 20° South and are crossed by lines indicating hours. To find the sun’s
position for the required conditions, select the point where the appropriate lines intersect. The sun’s
altitude (in degrees above the horizontal plane) is shown by the relation of this point to the concentric
circular lines within the diagram. The direction of the sun’s rays is shown by a line drawn through this point
from the outer graduated circle towards the centre.

Figure 10.4 Solar Chart for North Queensland

August 2001 10-7


Further emphasis of the importance of these Ideally vertical elements should be superimposed
elements of the design process is given in on horizontal ones in such a way that the
US DOT, 1997 as follows: intersection points practically coincide with the
horizontal slightly in advance of the vertical, and
“The interrelationship of horizontal and vertical the horizontal and vertical curves are of similar
alignment is best addressed in the route location lengths.
and preliminary design phases of the project. At
this stage, appropriate trade-offs and balances When horizontal and vertical curves are
between design speed and the character of the superimposed, the combination of superelevation

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road - traffic volume, topography and existing and vertical profile may cause distortion in the
development - can be made. … outer pavement edges which could confuse
drivers at night ie. give poor driver perception.
Because they must be complementary, horizontal Computer packages are available which enable a
and vertical geometry must be designed designer to view the roadway from the drivers
concurrently. Uncoordinated horizontal and perspective. The use of these packages can enable
vertical geometry can ruin the best parts and a designer to identify such distortions in the outer
accentuate the weak points of each element. pavement edges.
Excellence in the combination of their designs
increases efficiency and safety, encourages Figure 10.5 shows examples of ideal and
uniform speed, and improves appearance - almost acceptable co-ordination of horizontal and
always without additional cost.” vertical curves.

It is therefore essential that the designer establish
these elements of the design in the concept stage, 10.3.3 Specific Co-ordination
confirm them in the planning stage and properly Issues
detail them in the design stage. They must be
addressed in the earliest stages of any new road Some situations in which poor coordination of
development proposal, and maintained vertical and horizontal alignment occurs are
throughout the planning and design process. discussed in the following paragraphs.

The vertical curve overlaps one end
10.3.2 Principles of Co-ordination of the horizontal curve

Users see the road as a constantly changing three- If a vertical crest curve overlaps either the
dimensional continuum, and unless designers take beginning or the end of a horizontal curve, drivers
account of this fact, they may not appreciate how have little time to react to the horizontal curve
the finished design will appear to road users. once it comes into view. This is a particularly
Thus, horizontal and vertical alignments must not unsafe practice if there is a decrease in 85th
be considered in isolation. percentile speed at the start of the horizontal
curve. An example of this is shown in the upper
The driver sees a foreshortened and, thus, diagram of Figure 10.6.
distorted view of the road, and unfavourable
combinations of horizontal and vertical curves The defect may be corrected in both cases by
can result in apparent discontinuities in the completely separating the curves. If this is
alignment, even though the horizontal and vertical uneconomic, the curves must be adjusted so that
designs each comply separately with the they are coincident at both ends, if the horizontal
provisions of Chapters 11 and 12. curve is of short radius. If the horizontal curve is
of longer radius, they need be coincident at only
The main principle of obtaining good co- one end.
ordination between horizontal and vertical
alignments is given below:

10-8 August 2001


The vertical curve overlaps both ends Long Flat Grades
of the horizontal curve
Long straights with flat grades make it difficult
If a vertical crest curve overlaps both ends of a for drivers to judge the distance and speed of
sharp horizontal curve, a hazard may be created approaching vehicles leading to overtaking
because a vehicle has to undergo a sudden change accidents. An approaching vehicle more than
of direction during passage of the vertical curve 2500m away on a straight seems to be standing
while sight distance is reduced. This creates the still but the same situation on a large curve
same problems as discussed above. provides the driver with a changing perspective

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allowing some judgement of speed and distance.
The corrective action is to make the curves This situation is exacerbated at night.
coincident at one end so as to bring the crest on to
the horizontal curve. Roller Coaster Grading

Insufficient separation between the Long straight sections are prone to “roller
curves coaster” grading (see Figure 10.7) with the added
potential for hidden dips. Designers should take
If there is insufficient separation between the ends
care that these features are not incorporated into
of the horizontal and vertical curves, a false
the design by using appropriate curvature in both
reverse curve may appear on the outside edge-line
planes and checking lines of sight for hidden dips.
at the beginning of the horizontal curve, or on the
inside edge-line at the end of the horizontal curve.
Optical Summit
This is a visual defect and is illustrated in the
middle diagram of Figure 10.6. Figure 10.8 illustrates a phenomenon known as
“Optical Summit” brought about by the
Corrective action consists of increasing the
combination of vertical and horizontal
separation between the curves.
alignments. The horizontal curve does not overlap
the two vertical curves and the vertical curves are
Dissimilar length horizontal and
too small for the large movements in the other
vertical geometric elements
plane. The appearance has been improved
A short movement in one plane should not be considerably by the adjustments to both
placed within a large movement in the other. A alignments as shown in the figure. The horizontal
particular instance where this can lead to safety curve has been lengthened to encompass both of
problems is when a small depression in the the sag vertical curves and the sag curves have
vertical alignment results in a ‘hidden dip’. An been lengthened to be more in keeping with the
example of a hidden dip is shown in the upper scale of the other movements.
diagram of Figure 10.9. (See also Figure 10.7.)
This example also illustrates the benefit of using
Large depressions such as those shown in the
perspective views during the design process to
lower diagram of Figure 10.5 are not deemed to
highlight potential problems while they can still
be hidden dips.
be corrected at minimal cost.
Corrective action consists of making both ends of
horizontal and vertical curves coincident, thus
producing similar length curves. An alternative 10.4 Fitting the Road to the
treatment is to completely separate the curves.
Terrain
Figures 10.9 and 10.10 show examples of grossly
dissimilar lengths of both horizontal and vertical 10.4.1 General
geometric elements where short movements in
one plane are combined with large movements in A visually attractive and physically unobtrusive
the other. road will be one that integrates well with the
environment through which it passes.

August 2001 10-9


“A general rule for designers is to achieve a designed to match the terrain, the road will be
flowing line, with a smooth and natural consistent with driver expectations, aesthetically
appearance in the land, and a sensuous, rhythmic pleasing and economical. Figure 10.12 shows two
continuity for the driver. This effect results from alignments through the same hill, one generally
following the natural contours of the land, using following the contours and producing a more
graceful and gradual horizontal and vertical pleasing result than the other at a much lower cost
transitions, and relating the alignment to in earthworks.
permanent features such as rivers and mountains.”
(Aesthetics in Transportation, US DOT, 1980). The use of continuous liberal curves is preferred

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to long tangent, short curve design. An example of
The opposite of this occurs with designs using a this is shown in Figure 10.5.
combination of long tangents with short curves
whether in the horizontal or vertical plane.
Figures 10.10 and 10.11 illustrate examples of the
results achieved.

In urban areas, the type of surroundings will
depend on the land use of the areas through which
the alignment passes. The hard lines of an
industrial area will be complemented more by a
design which incorporates features such as
retaining walls and other structures more in
keeping with that urban form. The urban form of
the area being traversed will dictate the detail of
the design whatever that form. A length of urban
road may call for several changes in the design
concept as the road passes from one form to
another.

Varying the width and slope of batters

In flat open terrain, long straight road sections are
common, but generally there is advantage in
avoiding excessive lengths of straight road. A
gentle curvilinear design (consisting of large radii
horizontal curves) always helps to keep the
Split carriageways in urban setting operating conditions ‘under control’ and at the
same time, affords scope for far more sympathetic
In other areas it will be rare that the terrain fitting of the road to the terrain.
precisely matches the speed that must be used for
the design of the alignment elements. Earthworks In addition, safety is enhanced by making the
will be needed to mould the road into the terrain drivers more aware of their speed, by allowing
but careful attention to the principles in this them to make better assessment of the distances
Chapter will minimise the intrusion caused by this. and speeds of other vehicles, by reducing
headlight or sun glare in appropriate
It was pointed out in Section 6.1 that drivers will circumstances, and by reducing boredom and
adopt a speed related to the horizontal alignment fatigue.
and the terrain. If the horizontal alignment can be

10-10 August 2001


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Figure 10.5 Ideal and Acceptable Co-ordination of Vertical and Horizontal Alignments

August 2001 10-11


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Figure 10.6 Poor Co-ordination of Vertical and Horizontal Alignments

10-12 August 2001


Plan

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Profile

Hidden Dips

Perspective

880 CLP

Figure 10.7 Roller Coaster Grading resulting in hidden dips

August 2001 10-13


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Figure 10.8 The “Optical Summit” Effect

10-14 August 2001


For additional information on curvilinear
alignment design in flat terrain, refer Section
10.11.

10.4.2 Approaches to Design

Designers have to exercise their creativity and
ingenuity as well as their knowledge and skill in

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road design to produce the most effective results.
Some of the ways in which they can improve the
likelihood of more pleasing and effective designs
are discussed below. Blending batters with the roadside

• Careful attention to detail will alert the designer • Use of a multidisciplinary team will assist the
to potential improvements in the design - designer to reach the right conclusions. The
- Minor alignment shifts team must be in place at the start of the process
to achieve the best result (see 10.1 above). For
- Rounding of batters, shoulders and table the most complex designs, in addition to the
drains road designer, the team may include -
- Flattening fill and cut batters, particularly - Landscape Architect
where they are small (may also be able to
avoid the use of guard rail) - Urban Designer/ Architect

- Adopting constant minimum batter catch - Environmental Scientist
points to transition from cut to fill - Town Planner
- Using independent alignment and grading - Structural Engineer
for each direction of traffic - Road Engineers (drainage, pavement,
- Cut and cover tunnels to overcome both geotechnical)
urban and rural problems of separation

Location principles - choice of alignment

August 2001 10-15


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Figure 10.9 Poor Co-ordination of Alignments

10-16 August 2001


An example of visually poor alignment with unrelated horizontal and vertical curves, and broken backed
horizontal curves
10

A short movement in one plane should not be placed within a large movement in the other

Long straight grades combined with short vertical curves on a straight alignment produce a visually poor
result

Figure 10.10 Poor Co-ordination of Alignments

August 2001 10-17


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Figure 10.11 Fitting the Road to the Terrain

10-18 August 2001


For smaller projects, the size and expertise of asleep.
the team would have to be tailored to the needs
of that project. However, most projects will This can be explained by considering the effect of
need to have input from many of these speed on perception and vision. As speed
specialists in any case and the designer should increases -
ensure that the appropriate questions are put to • concentration increases;
them and suitable solutions generated.
• the point of concentration recedes;
• Preservation of / Integration with natural and
• peripheral vision diminishes;

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man made features can enhance the fit of the
road to the landscape and capture notable • foreground detail begins to fade; and
points of interest for the driver and passengers
• space perception becomes impaired.
of the vehicles using the road. And it will
reduce the intrusion of the road into the Thus the higher the speed, the further ahead the
landscape. These features can often create an driver focuses and the more concentrated the
“aiming point” to justify a change in direction angle of vision becomes. This restriction of vision
of the road thereby introducing interest into (sometimes called “tunnel view”) is the process
what may otherwise be a boring vista. Such used to induce hypnosis and sleep. Unless the
features include - point of concentration is made to move around
- Unique rock formations laterally by means of a curvilinear layout of the
- Isolated hills / hills of interesting shapes road, driving will become hypnotic causing the
driver to fall asleep.
- Patches of rain forest
- Feature trees (or clumps of trees) To achieve space perception, it is necessary to
have a lateral component of movement to allow
- Streams and lakes
the driver to assess relative changes in the size and
- Man made features (historic building; position of objects. This enables the driver to
isolated property buildings; windmills) discern movement and its direction. Horizontal
curves provide this lateral component, the rate of
Figure 10.12 illustrates examples where the road
movement depending on the radius of the curve.
has been designed sympathetically with the
landscape compared with the alternatives which The concept of a continuously curving alignment
ignored the impact on the landscape. with constant, gradual and smooth changes of
direction evolved and the curvilinear alignment
concept of long flat circular curves joined by
10.5 Curvilinear Design in spiral transitions was born. Inherent in this
Flat Terrain concept was the idea that curves would vary up to
30,000m radius. If the curve radii are confined
The traditional approach to the design of roads on within the limits of 10,000m to 30,000m, the need
extensive flat plains has been to use long tangents for transitions disappears.
with relatively short horizontal curves between
them. In some cases, this has created
exceptionally long lengths of straight resulting in
monotonous driving conditions.

While the straight sections provide some
orientation, the road is uninteresting as it is totally
predictable. The constant sameness of view makes
driving monotonous and fatiguing leading to the
driver losing all sense of pace, time and real speed
with the distinct possibility of the driver falling

August 2001 10-19


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A shorter route, but more cutting and
a scarred landscape

A longer route, but less cutting
and a road blended to the land.

A skyline cutting, straight
alignment with vertical curve

Re-aligned to fit landscape;
additional excavation on
road side to eliminate
residual mound.

Figure 10.12 Examples of Road-landscape Compromise

10-20 August 2001


angle as a minimum resulting in curve radii
between 3000m and 30,000m. Where extensive
flat plains occur, it is desirable that overtaking
sight distance is provided on as much of the road
as possible. This can be achieved on a 15 000m
radius curve, and an alignment with this radius as
a minimum will have continuous overtaking sight
distance.

10
The larger the radius, the closer the alignment
becomes to a straight line and the less the
advantages of the curve become. There is little
point in using radii larger than 30 000m for this
reason.

Curvilinear alignment from the driver’s viewpoint Table 10.1 Curvilinear Alignment - Minimum
Requirements Summary

Factor Value
Radius • Min. 3000m; Max. 30,000m
Deflection • Min. 2° for visible part of the
curve (e.g. for 15,000m radius,
a length of 524m is required).
• Desirable 3° (requires curves
from 3000m to 30,000m radius).
• Over 30,000m, the road
appears straight - there is no
point in using curves of larger
radius than this.
Overtaking • 15,000m radius curves provide
Sight Distance continuous overtaking sight
distance.

The advantages of a curvilinear alignment are:
Typical long tangent from the driver’s viewpoint
• driving is more pleasant as the road unfolds
itself smoothly with no unexpected checks;
The radius to use depends on several factors • judging distance to an approaching vehicle is
including the type of topography and the driver's easier for the driver and therefore better
desired speed of travel, the preferred radius decisions on overtaking manoeuvres can be
depending on how far ahead the driver can see the made;
road. At high speeds, the driver looks from 300m
to 600m ahead and a curve should be at least this • the view ahead is constantly changing;
long to be visually significant when the driver is • monotony can be reduced by directing the road
on it. A curve ceases to be visually significant if towards interesting features of the countryside
the visible part of the curve does not accomplish a for short periods; and
turn of more than two degrees (e.g. for a curve of
radius of 15,000m, the visible length of roadway • night driving is more comfortable in that the
should be at least 524m). approaching headlight glare is significantly
reduced. On long straights, headlights become
It is desirable to adopt a three-degree deflection visible and annoying from a long distance away

August 2001 10-21


(up to 15km), but on a curvilinear alignment, appearance of kinks. Radii in both dimensions
the glow of the approaching headlights will have to be tailored to meet the circumstances,
becomes visible before the lamps become based on the extent of the driver's vision of the
visible, the rate of approach can be better road ahead.
judged and the disability glare is significantly
delayed. The principles of curvilinear alignment can be
applied in a wide range of circumstances using a
Where the road passes through a forested area, the wide range of curve radii together with spirals.
curvature of the road creates a continuous The vast plains of the inland lend themselves to

10
backdrop of trees providing excellent conditions this approach and roads designed on these
for detecting the continuing angular change. The principles provide an improved result for the
larger radii alignments have worked well in these users.
cases. On treeless plains, some of the effect of the
curvilinear alignment is lost. In these
circumstances, the smaller radii would be more References
effective.
AASHTO (1994): “A Policy on the Geometric
Design of Highways and Streets”.

MacPherson, R.A. and L.J. Louis (1976):
“Curvilinear Alignment Design in the South
Western Plains” - Queensland Roads.

Tunnard, C. and B. Pushkarev (1963): “Man
Made America: Chaos or Control”.

U.S. Department of Transportation (US DOT)
(1980): “Aesthetics in Transportation”.

U.S. Department of Transportation (US DOT)
(1997): “Flexibility in Highway Design”.

Relationship to Other
Chapters
• Closely related to Chapters 11 and 12 – they
provide the details, this chapter provides the
New large radius curvilinear alignment adjacent to guidelines for putting them together;
old, long tangent/short curve alignment
• The cross section and how it interacts with the
horizontal and vertical alignments also affects
In more undulating topography, the combination the operation and aesthetics (Chapter 7);
of the vertical curvature with the large radius
horizontal curves becomes important and the • The principles of this chapter have to be
principles discussed in 10.3 above must be considered in planning – Chapter 2;
applied. For example, short sag vertical curves
• Alignment issues arise in Chapters 4, 13, 14,
may be accentuated by the large radius horizontal
15, 16, 20 and 22.
curves creating the appearance of a kink. It may
be necessary to adopt a vertical alignment with
significantly larger radius curves to avoid the

10-22 August 2001

Rpdm chapter10

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